[MOC] Mack Granite Boom Truck (INSTRUCTIONS AVAILABLE)

JLiu15 · June 15, 2023

Model of a Mack Granite truck equipped with a boom and flatbed. Features drive, steering, full suspension, motorized boom elevation, extension, and rotation, winch, and extending/lowering outriggers.

Functions/features:

Drive
Steering
Full suspension
Boom elevation
Boom extension
Boom rotation
Winch
Outriggers

Instructions available on Rebrickable: https://rebrickable.com/mocs/MOC-150955/JLiu15/mack-granite-boom-truck/#details

After the 4-output distribution gearbox on my MCI D4500CT Commuter Coach model proved to be ineffective, I didn't want to simply abandon the gearbox design. I decided that I was going to tweak the gearbox a little to be used in a new MOC. I had wanted to use the same suspension setup for my MCI D4500CT model as a test bed for the suspension design, so I settled on incorporating the gearbox into a truck model.

A boom truck like this is a MOC I had wanted to make for quite some time, as I see plenty of them in the US but not that many MOCs of them. Because the turntable for the boom is placed higher up, it leaves more room to route gears/axles from the gearbox to the 4 functions it controls. Because the gearbox sits in the heart of the chassis between the outriggers and rear axles, the chassis is built very densely. The gearbox makes use of two Control+ L motors: one for shifting and one for driving the output functions. The drive motor (a Control+ XL motor) sits directly behind the gearbox, and drive is transferred to the rear axles via an extendable CV joint. The drivetrain is geared down after the CV joint to reduce the amount of torque applied to it. Lastly, there is another Control+ L motor for steering which is built into the front axle. I designed the suspension (a simple live axle for all 3 axles) entirely in Stud.io before building it physically, so I had no idea how it would perform upon building it. However, the entire setup worked flawlessly, and there are no issues with drive transmission into the suspended rear axles.

My initial plan for this MOC was to just build it as a generic US-style truck, but upon finishing the cab someone said it resembled a Mack Granite, so I modified the cab to actually make it based on the Mack Granite with the Mack Bulldog emblem from the 42078 Mack Anthem set in the front. A number of metallic silver elements are used in the front, although due to the relatively low number of pieces available in that color there is still plenty of gray in the front (such as the bumper). The Control+ hub is placed in the back of the cab and is removable for battery replacement by removing the roof, as the hub and roof are attached to each other. This means the sides of the cab must be sturdy to compensate for the relatively loosely attached roof, so the doors do not open on this model. I decided to build the cab in dark blue, a color I have not used much before, as I now have a sizeable collection of dark blue pieces from the 42083 and 42154 sets. I built the boom in yellow to contrast with the dark blue cab.

The boom is built around 3x19 frames covered with tiles. Their abundance of pin holes makes them a good candidate for crane booms. It was difficult routing the two motorized functions through the turntable and into the boom, as two separate axle functions must pass through a small space. Because only two axle functions can be routed through a turntable, I had to make the winch manually operated with a ratchet mechanism. I was initially concerned about the weight of the boom, and while the boom does tend to lean forward during the elevation process, everything remains intact even at maximum elevation and extension and the range of the boom is quite impressive. There are no stoppers for the boom rotation, so the boom can theoretically rotate infinitely as there are no wires going through the turntable. The outriggers, however, were a weak point for the model as they are quite flimsy and are unable to lift the truck off the ground without substantial bending, so I limited the outriggers' legs extension to where they are able to make contact with the ground. Additionally, I should have geared down the mechanism substantially more, but a modification would require essentially rebuilding the chassis from scratch. As a result, I set the gearbox motor responsible for driving the gearbox functions slightly below maximum speed. I had wanted to be able to control the gearbox functions with a single touch (e.g. pressing one button shifts the gearbox then drives it in a given direction) but was not able to figure it out so I went with a much more straightforward (albeit more difficult to use) controller setup. A single slider is used to drive the gearbox functions, and three buttons are used to shift the gearbox (up 90 degrees, down 90 degrees, and return to one endpoint). I have to keep in mind the position of the gearbox while controlling the model, but the setup overall worked well.

Overall, this was a MOC that had plenty of mechanical complexity, but certain functions could've been designed better. While all of the functions work as intended, the boom leans forward quite a bit when the boom elevates and the outriggers cannot lift the truck off the ground. The front axle could've also been reinforced better, as during the steering calibration process the steering pinion gear often skips a tooth. Despite the drawbacks of this MOC, it still largely met my expectations as the drivetrain and suspension worked flawlessly and the dark blue/yellow color scheme worked well.

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